One frequent cause of a network fault is a short circuit occurring somewhere in the electricity distribution network. A short circuit leads directly to a local voltage dip. Within fractions of seconds, the voltage dip propagates over long distances through the electricity distribution network. The normal reaction to a network fault is to isolate the subelement in which the short circuit has occurred from the rest of the electricity distribution network. Before the subelement with the short circuit is isolated from the rest of the electricity distribution network, the electricity generators have no chance to stabilize the voltage. It is particularly important for the electricity generators not to be isolated from the electricity distribution network during this phase.
One critical factor for the further stability of the electricity distribution network is the phase shortly after the subelement with the short circuit has been isolated from the rest of the electricity distribution network. Although the voltage in the electricity distribution network then rises quickly again, it does not, however, rise immediately to the nominal voltage but first of all to a value of, for example, 75% of the nominal voltage. While the network is any case in a critical state because of the low voltage, control mechanisms can lead to the stabilization process being made even more difficult. This is true, for example, when the stepping switches on the load side change the step-up ratio to the high-voltage network, in order to allow the loads to be supplied with the nominal voltage again. The change in the step-up ratio results in an increase in the current in the high-voltage network, and therefore in a further fall in the voltage in the high-voltage network. Switching of the stepping switches on the load side therefore leads to a positive-feedback effect, which acts against complete reproduction and stabilization of the electricity distribution network. This positive-feedback (destabilizing) effect is particularly problematic because the synchronous generators which are responsible for the basic load are able only to a very restricted extent during this phase to provide the reactive current which is required to support the voltage.